1. Field of the Invention
The present invention relates to an oxide catalyst for use in catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase. More particularly, the present invention is concerned with an oxide catalyst for use in catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, which comprises, in a specific ratio, molybdenum (Mo), vanadium (V), antimony (Sb), niobium (Nb), oxygen (O) and at least one element Z selected from the group consisting of tungsten, chromium, titanium, aluminum, tantalum, zirconium, hafnium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, zinc, boron, indium, germanium, tin, lead, bismuth, yttrium, gallium, rare earth elements and alkaline earth metals, wherein the Sb/Mo atomic ratio (b) is larger than V/Mo atomic ratio (a), and the Sb/Mo atomic ratio (b) does not exceed 0.4. By the use of the oxide catalyst of the present invention in the oxidation or ammoxidation of propane or isobutane in the gaseous phase, (meth)acrylonitrile or (meth)acrylic acid can be produced with high selectivity and such high selectivity can be maintained for a long time, so that (meth)acrylonitrile or (meth)acrylic acid can be efficiently produced for a long time.
The present invention is also concerned with a process for producing an unsaturated carboxylic acid or an unsaturated nitrile in the presence of the abovementioned oxide catalyst.
2. Prior Art
Conventionally, there have been well known a process for producing (meth)acrylonitrile by ammoxidation of propylene or isobutylene, and a process for producing (meth)acrylic acid by oxidation of propylene or isobutylene. Recently, as substitutes for such processes for the ammoxidation or oxidation of propylene or isobutylene, attention has been attracted to a process for producing (meth)acrylonitrile or (meth)acrylic acid by a catalytic ammoxidation or oxidation in the gaseous phase, wherein propane or isobutane is used as a raw material instead of propylene or isobutylene. As catalysts for use in these processes, a number of catalysts have been proposed.
Of the catalysts proposed, especially, an oxide catalyst comprising Mo—V—Sb—Nb has been attracting attention, since such an oxide catalyst has advantages in that the catalyst comprises elements having a relatively low volatility, the catalyst can be used for a catalytic ammoxidation or oxidation in the gaseous phase at a low reaction temperature, and (meth)acrylonitrile or (meth)acrylic acid can be produced with relatively high selectivity and in relatively high yield.
Methods for producing (meth)acrylonitrile in the presence of the oxide catalyst comprising Mo—V—Sb—Nb (hereinafter, frequently referred to as an “Mo—V—Sb—Nb oxide catalyst”) are disclosed in various patent documents, such as Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-157241 (corresponding to U.S. Pat. No. 5,750,760 and EP 0767164 A1), 10-28862, 10-81660, 10-310539, 10-330343, 11-42434, 11-43314, 11-57479, 11-263745, 2000-1464, 2000-143244, WO 0012209 A1 (corresponding to DE 1998325 T), and U.S. Pat. No. 6,043,185.
Methods for producing (meth)acrylic acid in the presence of the Mo—V—Sb—Nb oxide catalyst are also disclosed in various patent documents, such as Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-316023, 10-118491, 10-120617 (corresponding to U.S. Pat. Nos. 5,994,580 and 6,060,422), 10-137585, 11-285637, 11-343261, 2000-51693, 11-343262, 10-36311, 10-45664, 9-278680 and 10-128112.
Each of the above-mentioned Mo—V—Sb—Nb oxide catalysts, which are used for producing (meth)acrylonitrile or (meth)acrylic acid, comprises an oxide represented by the following formula (a):Mo1VpSbqNbrOm  (a)                wherein p, q, r and m are, respectively, the atomic ratios of V, Sb, Nb and O, relative to Mo.        
The above-mentioned conventional Mo—V—Sb—Nb oxide catalysts can be categorized into the following two groups:    (i) catalysts in which the V/Mo atomic ratio is equal to or larger than the Sb/Mo atomic ratio, i.e., p and q in formula (a) above satisfy the following relationship: p≧q; and    (ii) catalysts in which the Sb/Mo atomic ratio is larger than V/Mo atomic ratio, i.e., p and q in formula (a) above satisfy the following relationship: p<q,
wherein the Sb/Mo atomic ratio is equal to or larger than 0.5, i.e., q in formula (a) above satisfies the following relationship: q≧0.5.
When the conventional Mo—V—Sb—Nb oxide catalysts mentioned above are used, (meth)acrylonitrile or (meth)acrylic acid is sometimes produced with a relatively high selectivity (hereinafter, (meth)-acrylonitrile or (meth)acrylic acid is frequently referred to as the “desired product”). However, the selectivity for the desired product, which is achieved by such conventional catalysts, is not satisfactory.
Of the Mo—V—Sb—Nb oxide catalysts of group (i) above, oxide catalysts capable of achieving a relatively high selectivity for the desired product exhibits a disadvantageously low stability. Specifically, especially when the catalytic oxidation or ammoxidation in the gaseous phase is performed in the presence of each of such oxide catalysts in a recycling mode using a gaseous feedstock mixture having a high partial pressure of propane, the selectivity for the desired product decreases with the lapse of time.
In an attempt to improve the stability of the Mo—V—Sb—Nb oxide catalysts of group (i) above so as to maintain the selectivity for the desired product at a high level, the following two methods have been proposed:    a first method in which, using a reactor having a zone in which a gaseous mixture having a higher oxygen concentration than that of a gaseous reaction mixture produced is contacted with the oxide catalyst, the oxide catalyst is continuously oxidized to regenerate the oxide catalyst (see Unexamined Japanese Patent Application Laid-Open Specification No. 11-263745); and    a second method in which an Mo—V—Sb—Nb oxide catalyst of group (i) above produced by a process comprising preparing a raw material liquid mixture for the catalyst, followed by spray-drying and calcination is mixed with an aqueous solution containing Mo and Co to obtain an aqueous mixture, and the obtained aqueous mixture is spray-dried and calcined to thereby obtain a modified catalyst containing a large amount of an Mo—Co composite oxide (see Unexamined Japanese Patent Application Laid-Open Specification No. 11-57479).
Of the above-mentioned two methods, the first method is disadvantageous in that the process for the catalytic ammoxidation or oxidation in the gaseous phase inevitably becomes cumbersome. On the other hand, the second method is also disadvantageous not only in that the process for producing the oxide catalyst becomes too cumbersome, but also in that, even when silica is used to increase the strength of the catalyst, since the oxide catalyst produced in the second method contains a large amount of an Mo—Co composite oxide, it is difficult to cause the oxide catalyst to contain a satisfactory amount of silica which is needed to satisfactorily increase the strength of the oxide catalyst. Therefore, especially, it is difficult to apply the second method to the production of an oxide catalyst which is used for a reaction in a fluidized bed and, hence, is required to have a high strength.
With respect to the Mo—V—Sb—Nb oxide catalysts of group (ii) above, these catalysts have a disadvantage in that the selectivity for the desired product is low.
From the above, it is apparent that, by the conventional Mo—V—Sb—Nb oxide catalysts for the catalytic ammoxidation or oxidation, it is difficult to stably produce the desired compound with high selectivity for a long time.